The present invention relates to a voltage detector for detecting the voltage developing in a selected area of an object to be measured such as an electric circuit. In particular, the present invention relates to a voltage detector of the type that detects voltage by making use of the change in light polarization that occurs in accordance with the voltage developing in a selected area of an object to be measured.
Various voltage detectors have been used to detect the voltage developing in a selected area of objects to be measured such as electric circuits. Conventional voltage detectors are roughly divided into two types: in one type, the probe is brought into contact with a selected area of an object to be measured and the voltage developing in that area is detected; and in the other type, the probe does not make contact with a selected area of an object to be measured and instead an electron beam is launched into that area and the voltage developing in it is detected.
Voltage changes rapidly in fine-line portions of objects such as integrated circuits that are small and complicated in structure, and a strong demand exists in the art for detecting such rapidly changing voltage with high precision without affecting the condition of the fine-line portions. However, this need has not been fully met by the prior art voltage detectors. With devices of the type that detects voltage by bringing the probe into contact with a selected area of an object to be measured, it is difficult to attain direct contact between the probe and a fine-line portion of the object of interest such as an integrated circuit. Even if this is successfully done, it has been difficult to correctly analyze the operation of the integrated circuit solely on the basis of the voltage information picked up by the probe. A further problem involved is that contact by the probe can cause a change in the operation of the integrated circuit. Voltage detectors of the type that employs an electron beam has the advantage that they are capable of voltage detection without bringing the probe into contact with an object to be measured. However, the area to be measured with such voltage detectors has to be placed in vacuum and its surface must be exposed at that. In addition, the area to be measured is prone to be damaged by the electron beam.
The prior art voltage detectors have a common problem in that they are unable to operate quickly enough to follow rapid changes in voltage and hence fail to achieve precise detection of voltages that change rapidly as in integrated circuits.
With a view to solving these problems, it has been proposed by the present inventors (Japanese Patent Application No. 137317/1987 filed on May 30, 1987) that voltage be detected by making use of the polarization of a light beam that changes with the voltage developing in a selected area of an object to be measured.
A voltage detector operating on this principle is schematically shown in FIG. 4. The detector generally indicated by 50 is composed of the following components: an optical probe 52; a CW (Continuous-Wave) light source 53 typically in the form of a laser diode; an optical fiber 51 for guiding a light beam from the CW light source 53 into an optical probe 52 through a condenser lens 60; an optical fiber 92 for guiding reference light from the optical probe 52 into a photoelectric converter 55 through a collimator 90; an optical fiber 93 for guiding output light from the optical probe 52 into a photoelectric converter 58 through a collimator 91; and a comparator circuit 61 for comparing the electric signals from the photoelectric converters 55 and 58.
The optical probe 52 is equipped with an electro-optic material 62 such as an optically uniaxial crystal of lithium tantalate (LiTaO.sub.3). The tip 63 of the electro-optic material 62 is worked into a frustoconical shape. The optical probe 52 is surrounded with a conductive electrode 64 and has at its tip 63 a coating of reflecting mirror 65 in the form of a thin metal film or a multilayered dielectric film.
The optical probe 52 further includes the following components: a collimator 94; condenser lenses 95 and 96; a polarizing beam splitter 56 that extracts from the light beam passing through the collimator 94 reference light having a predetermined linear polarization component and input light to be launched into the electro-optic material 62 having a linear polarization component perpendicular to that of the reference light, and also extracts output light having a predetermined polarized component from a light beam emerging from the electro-optic material 62. The reference light is passed through the condenser lens 95 and thence launched into the optical fiber 92, whereas the output light emerging from the electro-optic material 62 is passed through the condenser lens 96 and thence launched into the optical fiber 93.
Voltage detection with the system shown in FIG. 4 starts with connecting the conductive electrode 64 on the circumference of the optical probe 52 to a predetermined potential, say, the ground potential. Then, the tip 63 of the probe 52 is brought close to the object to be measured such as an integrated circuit (not shown), whereupon a change occurs in the refractive index of the tip 63 of the electro-optic material 62 in the probe 52. Stated more specifically, the difference between refractive indices for an ordinary ray and an extraordinary ray in a plane perpendicular to the light-traveling direction will change in the optically uniaxial crystal.
The light beam issuing from the light source 53 passes through the condenser lens 60 and is guided through the optical fiber 51 to be directed into the collimator 94, and then into the polarizing beam splitter 56 in the optical probe 52. Each of the reference light and the input light, which are produced by passage through the polarizing beam splitter 56, has an intensity of I/2. As already mentioned, the refractive index of the tip 63 of the electro-optic material 62 varies with the voltage on the object being measured, so the input light launched into the electro-optic material 62 will experience a change in the state of its polarization at the tip 63 in accordance with the change in the refractive index of the latter. The input light is then reflected from the reflection mirror 65 and makes a return trip through the electro-optic material 62, from which it emerges and travels back to the polarizing beam splitter 56. If the length of the tip 63 of the electro-optic material 62 is written as l, the state of polarization of input light launched into that material will change in proportion to the difference between refractive indices for the ordinary ray and the extraordinary ray and to the length 2l as well. The output light that is fed back into the polarizing beam splitter 56 after experiencing a change in the state of its polarization is changed from I/4 to (I/4)sin.sup.2 [(.pi./2)V/V.sub.0 ] in the polarizing beam splitter 56 before it is further fed into the photoelectric converter 58. In the formula expressing the intensity of output light emerging from the polarizing beam splitter 56, V is the voltage developing in the object to be measured, and V.sub.0 is a half-wave voltage.
In the comparator circuit 61, the intensity of reference light, or I/2, is compared with the intensity of output light, or (I/4)sin.sup.2 [(.pi./2)V/V.sub.0 ], in the form of electric signals produced from respective photoelectric converters 55 and 58.
The intensity of output light, or (I/4)sin.sup.2 [(.pi./2)V/V.sub.0 ], will vary with the change in the refractive index of the tip 63 of the electro-optic material 62 that occurs as a result of the change in voltage. Therefore, this intensity can be used as a basis for detecting the voltage developing in a selected area of the object to be measured, say, an integrated circuit.
As described above, in using the voltage detector 50 shown in FIG. 4, the tip 63 of the optical probe 52 is brought close to the object to be measured and the resulting change in the refractive index of the tip 63 of the electro-optic material 62 is used as a basis for detecting the voltage developing in a selected area of the object of interest. Therefore, the voltage developing in fine-line portions of a small and complicated object such as an integrated circuit which are difficult to be contacted by a probe or which cannot be contacted by the same without affecting the voltage being measured can be effectively detected by the detector 50 without bringing the optical probe 52 into contact with such fine-line portions.
As was described above, :n the voltage detector of FIG. 4, the reflection mirror 65 is provided on the end portion 63 of the electro-optic material 62. Therefore, the voltage detector suffers from a difficulty that if the surface of the reflection mirror 65 is not accurately vertical to the optical axis (central axis of the electro-optic material), then in the electro-optic material 62 the optical path of the input light beam is not coincided with that of the output light beam, so that the voltage cannot be detected with high accuracy. Therefore, in installation of the reflection mirror 65, formation of even a small angle of inclination with the optical axis is not permitted, and the end portion 63 of the electro-optic material 62 must be manufactured with extremely high precision.